A thermoplastic polyurethane is usually produced by reacting a polyol compound with a diisocyane and a chain extender and has a linear polymeric molecular structure having hard segment portions and soft segment portions.
In conventional thermoplastic polyurethanes, the polyol compound is usually selected from polyether diol compounds and polyester diol compounds. Currently, however, polycarbonate diol compounds draw attention as an important material for a new type of thermoplastic polyurethane having excellent heat resistance, hydrolysis resistance and weather resistance.
It is known, however, that the polycarbonate structure-containing thermoplastic polyurethanes are disadvantageous in that the thermoplastic polyurethane is too rigid and thus the flexibility and extendability of the thermoplastic polyurethanes are low in comparison with those of conventional polyurethanes, particularly polyether structure-containing polyurethanes. Also, the polycarbonate structure-containing thermoplastic polyurethanes exhibit high glass transition temperatures and have disadvantageous mechanical properties at low temperature. To solve the above-mentioned problems, an attempt was made to employ, as a polyol compound, a polycarbonate diol compound having ether groups introduced into the molecules of the polycarbonate diol compound, namely a polyethercarbonate diol compound.
The diol compounds for providing the diol component in the polyethercarbonate diol are selected from, for example, polymeric diols as disclosed in Japanese Unexamined Patent Publication No. 59-66577, including mixtures of diol compounds comprising, as principal structures, polycarbonate chains, particularly 1,6-hexanediol polycarbonate glycol, with diol compounds containing ethylene oxide structure units, and block copolymeric compounds comprising, as principal components, polycarbonate chains and ethylene oxide structure units contained in one and the same molecule; polyether diols obtained by etherifying 1,6-hexanediol, as disclosed in Japanese Unexamined Patent Publication No. 63-305,127; and mixtures of polyether polyols, for example, diethylene glycol, triethylene glycol, tetraethylene glycol, tripropylene glycol, polypropylene glycol, and polytetramethylene glycol with polyhydric alcohols, for example, ethylene glycol, 1,2-propanediol, 1,3-butanediol and 1,6-hexanediol, as disclosed in Japanese Unexamined Patent Publication No. 2-255,822.
The conventional polyethercarbonate diols produced by using the above-mentioned polyether diol compounds are disadvantageous in that they are in the state of a solid at room temperature or are very viscous liquids having a high viscosity, and are very difficult to handle. Further, the conventional polyethercarbonate diols exhibit a glass transition temperature which is not sufficiently low, and thus the resultant polyurethanes are unsatisfactory in flexibility, mechanical properties at low temperature, and/or stretchability (elongation and recovery).